Part 3 – ETC
(Electron Transport Chain)
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v NAD and FAD are used to capture H-atoms
to become NADH2 and FADH2 . This is called oxidation. NADH2
and FADH2 will donate their H-atoms in
mitochondrion and become NAD and FAD again . This is called reduction. This is
done for two purposes. (1) The regained NAD and FAD can be used again to keep the
Glycolysis and the TCA-cycle running. (2) The H-atoms will go into the ETC
(Electron Transport Chain) to generate ATP (biochemical energy) for yeast growth and multiplication.
v Hydrogen (H)
Hydrogen atom is the
only chemical element without neutrons.
It contains a single positively charged proton (H+) and a single negatively charged electron(e-). It is the lightest element and is the
most abundant chemical substance in the universe. Hydrogen plays a particularly
important role in oxidation and reduction reaction in wine chemistry.
v ETC
(Electron Transport Chain)
ETC is a series of protein complexes embedded in the
mitchondrial inner membrane. It works as
following :
NADH2 donates the 2H to complex 1. The coenzyme flavin mononucleotide (FMN) is a stronger oxidizing
agent than NAD. NADH reductase is the 2 protons (H+) acceptor
and iron-sulfur
protein (FeS) is the 2 electrons (e– ) acceptor. The protein CoQ
(ubiquinone) transports these electrons to complex 3, and the protein cyt C (cytochrome
C) transports them to complex 4. Finally these electrons will be captured by
the oxygen (O), which is obtained by the cellular respiration. The oxygen
works here as an electron acceptor (oxidizing agent). Coupled with each electron transport, one
hydrogen ion (H+) is pumped by the complex from the matrix to the
intermembrane space . There are 2 electrons and
3 complexes. These 6 H+ increase the H+
concentration in the intermembrane space. That means the H+
concentration in the intermembrane space is higher than that in the matrix. This is called pH gradient. By a pH gradient, H+ will diffuse from an area of high
concentration to an area of lower concentration. It is called chemiosmosis. By this way the
H+ comes
back into the matrix through the ATP synthase, which can synthesize 3 ATP. The 6 returned H+ will then
be connected with the O and the e– to
form water : 2 H+ + 2 e- + 1/2 O2 ---> H2O, which will be
reused in the TCA-cycle.
FADH2 donates its 2 hydrogen atoms by complex 2, Succinate
dehydrogenase, which does not pump H+. Coupled with the electron
transport, 4 H+ are pumped
through. That’s why FADH2 gives only 2 ATP.
v Cellular
respiration
Yeast cells set off CO2 and
takes in O2 . This is cellular respiration, with objective to
transform glucose into ATPs.
v Recapitulation EMP,CAC & ET C
Wine yeasts are
aerobic and anaerobic. That means they can grow with or without oxygen.
In grape juice they use sugars to grow. In the presence of oxygen they
transform one sugar in cytoplasm and
mitochondia to yield 38 ATP and the growth is optimum. In the absence of
oxygen the cellular respiration will
stop. Pyruvate will not go into mitochondia. In this case pyruvate will
be decardoxylated and reduced to ethanol. The main purpose of this reaction is
to regenerate NAD which is needed for
the glycolysis. The net gain is 2
ATP and the growth is minimum.
v Oxidation & Reduction in wine chemistry
l In oxidation, molecule A
is oxidized by oxidising agent
NAD. It loses 2H and becomes molecule B.
Oxidation is
the loss of hydrogen.
l In reduction, molecule B
is reduced by reducing agent
NADH+H. It gains 2H and becomes molecule A.
Reduction is
the gain of hydrogen.
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P.S.
We have seen how and why alcohol is formed. However,
alcoholic fermentation produces not only ethanol, but also several other
compounds like glycerol, fatty acids, higher alcohols, esters, diacetyl, etc. Fortunately!
Without them, wine would have little organoleptic interest.
Coming next month, in Part 4, we’ll see why one 6-carbon
molecule
of glucose, after phosphorylation, is not split into two 3-carbon
molecules of the same structure but of 2 different
structures, and even stranger not in equal proportion but in a ratio of 96% to
4%. Also we’ll see how an why glycerol is
synthesized?
Fascinating how things work out in the nature.